Towson pen may also be attributed to effects 
mediated through the pituitary-adrenal system. 
B. Effect of Litter Size on Growth and Later Behavior. 
Seitz (59) has demonstrated some important effects 
of litter size. Fischer strain albino domesticated 
rats of the F 50 generation of inbreeding were used 
as subjects. Litter size was artificially determined. 
The experiment included 5 litters of size 12 and 10 
litters of size 6. Mothers of the smaller litters 
were more maternal in the sense that when dis- 
turbed they were more reluctant to leave the litter 
following disturbance, they were more diligent in 
searching for removed young, and upon finding 
them were more effective in retrieving them, and 
they built better nests. No observations were 
made on the interactions among siblings prior to 
weaning, although it was the initial assumption 
that the greater competition among sibs of litter 
size 1 2 for the opportunity of nursing would be the 
major cause of any postweaning differnces in 
physiology or behavior. At 21 days of age each 
of the 120 rats was placed in an individual cage 
where it remained in isolation except for those 
short tests, such as mating, that involved another 
individual. The following are typical of the types 
of results obtained having statistical significance 
at the 0.05 level or below. Males but not females, 
of the smaller litters grew more rapidly. Members 
of the smaller litters hoarded less, ate more follow- 
ing 23 hours of fasting, defacated less and traveled 
more when in a strange open field, and had a 
shorter latency in entering a strange field. In 
addition, there was the qualitative difference that 
members of smaller litters were more docile and 
never bit the handlers as did the members of 
larger litters. Although the exact effect of maternal 
behavior and preweaning sibling competition in 
producing the observed changes among Seitz’s 
rats cannot be stated, it is quite apparent that these 
variables were in operation in my free-ranging 
colony of wild Norway rats. Litters did vary in 
size and there was an indication in the case of 
females 43 and her litter, L-18, that some control 
of litter size may be exercised. It will be recalled 
that 6 of the 12 young born disappeared during the 
first 24 hours. There was also considerable in- 
direct evidence that the socially lower ranking 
females exhibited reduced maternal behavior. 
At least they more frequently failed to rear litters, 
and failed to exhibit the normal pattern of nest and 
burrow building associated with paturition. Fe- 
males who reared their progeny successfully did 
differ in nonmaternal behavior as well as in that 
constellation of characteristics which I have termed 
social rank. It is difficult to believe that such 
differences were not mirrored in maternal behavior 
as well, and that one of the major origins of differ- 
ences in postweaning behavior and physiology of 
rats resulted from the behavior of their mother. 
Following 16 months of isolation each of Seitz’s 
males was placed with a female in heat. In general 
mounting failed to occur. “Courting” in the form 
of grooming the female or licking her genitals did 
take place. Although members of large litters 
exhibited a longer latency in approaching the 
female (i.e. she was a strange object), they spent 
more time courting her. This courting involved 
oral behavior upon the part of the males. It was 
Seitz’s inference that the members of larger litters 
experienced more frustration in the early behavior 
of nursing, and the increased later hoarding of food 
pellets tends to confirm this inference. Grooming 
and genital licking are similar in their motor aspects 
to early behavior of nursing. Since nursing 
frustration augments even the more divergent oral 
behavior of hoarding, it is logical to expect an 
increase in the prevalence of grooming and genital 
inspection by the more frustrated males from large 
litters. This is my interpretation, whose origin is 
based on the previously cited observation for the 
wild Norway rats which showed that genital in- 
spection of the mother who is coming into heat at 
the termination of lactation is a direct transfer of 
the behavior of inspecting the teats by young who 
at weaning still trail their mother. 
C. Size of Group. Several lines of evidence from 
my experimental population were cited which 
indicated that as a general rule the ingroup rarely 
exceeded 10 or 12 rats. That is, the number of 
rats which lived in the same burrow or in closely 
neighboring burrows and had more frequent 
associations with each other than with other 
members of the population roughly approximated 
the usual litter size. This was so even though each 
group as finally constituted was composed of 
members of several different litters. From this I 
infer that tolerance of group size is determined by 
the number of individuals, mostly sibs, with which 
each individual had associations up to the time of 
weaning. It is presumed that whenever the group 
exceeds 10 individuals, antagonism between indi- 
viduals increases to the point that some individuals 
are excluded. Elton and Laurie [in (31)] provide 
a large group of data which supports this belief. 
261 
